Biometric image processing apparatus and biometric image processing method

ABSTRACT

A biometric image processing apparatus includes an imaging device that captures a first image of a biometric part that is irradiated with light having a first wavelength, and a second image of the biometric part irradiated with light having a second wavelength shorter than the first wavelength, and a computing device that computes a third image by subtracting from the first image an image in which a luminance of the second image is attenuated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2016-062462, filed on Mar. 25,2016, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are related to a biometric imageprocessing apparatus and a biometric image processing method.

BACKGROUND

Biometric authentication (or biometrics authentication) identifies auser using biometric features of the user, such as fingerprint, face,vein, or the like of the user. Vein authentication captures subcutaneousvein patterns to acquire the biometric features. Because the veinauthentication identifies the user using internal information, the veinauthentication provides an authentication accuracy that is highercompared to those provided by authentications that identify the userusing information such as the fingerprint, face, or the like.

In order to perform the vein authentication, light is irradiated on abiometric part to capture the vein pattern. For example, the lightirradiated on the biometric light may be near-infrared light thatpenetrates skin and reaches inside the biometric part. The biometricpart, such as a finger, hand, or the like, has a multi-layered structureincluding skin and internal structure. Hence, an appearance of thebiometric part may be represented by a dichroic reflection model. Inother words, in a case in which light is irradiated on the biometricpart, returning light from the biometric part is a mixture of lightreflected at a surface of the biometric part (that is, skin surface) andlight scattered inside the biometric part. Because veins are locatedunder the skin, an image caused by surface reflection is eliminated, andonly an image caused by internal scattering is obtained, in order toobserve the veins with a high accuracy.

The image caused by surface reflection and the image caused by internalscattering may be separated using polarization properties. For example,Japanese Laid-Open Patent Publication No. 2002-200050 proposes atechnique that uses a polarization filter to eliminate effects ofsurface reflection. The light reflected at the surface of the biometricpart maintains the polarization state thereof. On the other hand, thelight scattered inside the biometric part randomly change thepolarization state thereof. For this reason, when the polarizationfilter is arranged at a subsequent stage of an illumination end and at apreceding stage of an observation end so that the polarizationdirections at the illumination end and the observation end becomeparallel, the polarization filters cut an internally scattered lightcomponent, and a surface reflection light component can be observed by adetector. On the other hand, when the polarization filter is arranged atthe subsequent stage of the illumination end and at the preceding stageof the observation end so that the polarization directions at theillumination end and the observation end become perpendicular, thepolarization filters cut the surface reflection light component, and theinternally scattered light component can be observed by the detector.

However, the polarization filter transmits only the light having aparticular polarization direction, and cuts light having polarizationdirections other than the particular polarization direction byreflecting or absorbing the light having polarization directions otherthan the particular polarization direction. Consequently, an amount oflight that can be received by the detector via the polarization filterdecreases, and image noise increases, thereby making it difficult toobtain a clear biometric image. In addition, a polarization filter thatcan be used in a near-infrared range is expensive. As a result, whenproviding the polarization filter that can be used in the near-infraredrange, a cost of the biometric image processing apparatus increases.

Therefore, it is difficult to obtain a clear biometric image by aconventional biometric image processing apparatus.

Examples of related art include Japanese Laid-Open Patent PublicationsNo. 2002-200050, No. 2007-323389, and No. 2009-028427, Takaaki Maeda etal., “Monte Carlo Simulation of Spectral Reflectance Using aMultilayered Skin Tissue Model”, Optical Review Vol. 17, No. 3, (2010),pp. 223-229, and Yoshinaga Aizu, “Skin Tissue Multilayered StructureModeling and Light Propagation Simulation”, Journal of the Japan Societyof Mechanical Engineers (JSME), 2011.7, Vol. 114, No. 1112, p. 39.

SUMMARY

Accordingly, it is an object in one aspect of the embodiments to providea biometric image processing apparatus and a biometric image processingmethod, which can obtain a clear biometric image.

According to one aspect of the embodiments, a biometric image processingapparatus includes an imaging device configured to capture a first imageof a biometric part that is irradiated with light having a firstwavelength, and a second image of the biometric part irradiated withlight having a second wavelength shorter than the first wavelength; anda computing device configured to compute a third image by subtractingfrom the first image an image in which a luminance of the second imageis attenuated.

The object and advantages of the invention will be realized and attainedby means of the elements and combinations particularly pointed out inthe claims.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory and arenot restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating an example of a biometric imageprocessing apparatus in one embodiment;

FIG. 2 is a diagram for explaining acquisition of images by internalscattering;

FIG. 3 is a diagram illustrating an example of the biometric imageprocessing apparatus in a first embodiment;

FIG. 4 is a diagram illustrating an example of the biometric imageprocessing apparatus in a second embodiment;

FIG. 5 is a diagram illustrating an example of a filter;

FIG. 6 is a diagram illustrating an example of the biometric imageprocessing apparatus in a third embodiment;

FIG. 7 is a diagram illustrating an example of the biometric imageprocessing apparatus in a fourth embodiment;

FIG. 8 is a diagram illustrating an example of an RGB filter;

FIG. 9 is a diagram illustrating an example of a multi-layered structureof skin; and

FIG. 10 is a diagram illustrating an example of a light penetrationrate.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will be described withreference to the accompanying drawings.

A description will now be given of the biometric image processingapparatus and the biometric image processing method in each embodimentaccording to the present invention.

FIG. 1 is a diagram illustrating an example of the biometric imageprocessing apparatus in one embodiment. A biometric image processingapparatus 1 illustrated in FIG. 1 includes a light source 11, a camera12, and a control device 13.

The light source 11 irradiates illumination light on a biometric part500 that is an example of a capture (or imaging) target. The lightsource 11 irradiates light having two or more mutually differentwavelengths. The light source 11 may be formed by a single light sourcethat irradiates light having wavelengths including both long-wavelengthand short-wavelength, as described later in conjunction with FIGS. 3 and4. In addition, the light source 11 may be formed by a plurality oflight sources, such as a first light source that irradiates light havinglong-wavelength and a second light source that irradiates light havingshort-wavelength, as described later in conjunction with FIGS. 6 and 7.The light having the long-wavelength is an example of light having afirst wavelength, and the light having the short-wavelength is anexample of light having a second wavelength shorter than the firstwavelength. The light having the long-wavelength has a wavelength longerthan 600 nm, for example, and the light having the short-wavelength hasa wavelength of 600 nm or shorter.

The camera 12 receives returning light from the biometric part 50 thatis irradiated with the illumination light, and is an example of animaging device (or imaging means) that captures an image of thebiometric part 500. The camera 12 captures a first image of thebiometric part 500 irradiated with the light having the long-wavelength,and a second image of the biometric part 500 irradiated with the lighthaving the short-wavelength. In this example, the biometric part 500 isa palm of a person who is a target to be identified, and the camera 12captures an image of the palm.

The control device 13 includes a controller 131 that controls the lightsource 11 and the camera 12, and a computing device 132 that performs acomputing process on the image captured by the camera 12. The controldevice 13 may be formed by a processor, such as a CPU (CentralProcessing Unit) or the like. The control device 13 may include a memory(not illustrated) that stores, among other things, a program executed bythe processor. In this example, the computing device 132 computes athird image by subtracting from the first image of the biometric part500 captured by the camera 12 an image in which a luminance of thesecond image of the biometric part 500 captured by the camera 12 isattenuated. An attenuation rate at which the luminance of the secondimage is attenuated may be determined according to an intensity of acomponent of surface reflection light included in the first image. Inthis example, the computing device 132 obtains an image of veins of thepalm, or an image of a palm vein pattern, from the image of the palmcaptured by the camera 12.

In FIG. 1, the biometric part 500, that is an example of the capturetarget, is located above the biometric image processing apparatus 1.However, a position of the biometric part 500 with respect to thebiometric image processing apparatus 1 is not limited to a particularlocation. In addition, the position of the biometric part with respectto the biometric image processing apparatus is also not limited to aparticular location in each of the embodiments described later inconjunction with FIGS. 3, 4, 6, and 7. In FIGS. 1, 3, 4, 6, and 7,dotted-line arrows represent paths of the illumination light and thereturning light.

FIG. 2 is a diagram for explaining acquisition of images by internalscattering. For the sake of convenience, FIG. 2 illustrates contours ofthe palm with respect to each image. In FIG. 2, a first image 50-1 isthe image of the biometric part 500 irradiated with the light having thelong-wavelength and captured by the camera 12. In the first image 50-1,a bold solid line indicates a pattern 501 on a biometric surface of thebiometric part 500, such as a wrinkle or the like on a skin surface ofthe palm. In addition, a bold dotted line in the first image 50-1indicates a vein pattern 502 under the skin. Accordingly, the firstimage 50-1 includes information of the skin surface of the biometricpart 500, and subcutaneous information of the biometric part 500. Asecond image 50-2 is the image of the biometric part 500 irradiated withthe light having the short-wavelength and captured by the camera 12. Inthe second image 50-2, a bold solid line indicates the pattern 501 onthe biometric surface of the biometric part 500, such as the wrinkle orthe like on the skin surface of the palm. Accordingly, the second image50-2 mainly includes the information of the skin surface of thebiometric part 500. A third image 50-3 is the image obtained bysubtracting from the first image 50-1 the image in which the luminanceof the second image 50-2 is attenuated. In the third image 50-3, a boldsolid line indicates the vein pattern 502 under the skin.

This embodiment separates a surface component and an internal componentof the image by utilizing a depth of light, that penetrates the surfaceof an object and reaches inside the object, and differs depending on thewavelength of the light. Light more easily scatters as the wavelengththereof becomes shorter, and light more easily penetrates the surface ofthe object and reaches inside the object as the wavelength thereofbecomes longer. In a case in which the capture target on which the lightis irradiated is a biometric part, the light also more easily penetratesthe skin and reaches inside the biometric part as the wavelength of thelight becomes longer. Accordingly, light having the long-wavelength andlight having the short-wavelength are simultaneously irradiated orsequentially irradiated on the biometric part, in order to observe theimage for each wavelength.

Of the returning light from the biometric part, the long-wavelengthcomponent penetrates the skin, reaches inside the biometric part, andscatters inside the biometric part, and for this reason, the intensityof the long-wavelength component decreases. On the other hand, of thereturning light from the biometric part, the short-wavelength componentis reflected at the skin surface of the biometric part, and a decreasein the intensity of the short-wavelength component is small compared tothat of the long-wavelength component. Hence, the third image caused byinternal scattering of the biometric part, that is, the image of thevein pattern, is obtained by subtracting from the first image caused bythe light having the long-wavelength, the image in which the luminanceof the second image caused by the light having the short-wavelength, isattenuated. In other words, when obtaining the third image from adifference between the first image and the second image, the differenceis obtained by first adjusting the luminance of the first image and thesecond image.

FIG. 3 is a diagram illustrating an example of the biometric imageprocessing apparatus in a first embodiment. In FIG. 3, those parts thatare the same as those corresponding parts in FIG. 1 are designated bythe same reference numerals, and a description thereof will be omitted.In this embodiment, the light having the long-wavelength and the lighthaving the short-wavelength are simultaneously irradiated on thebiometric part, in order to observe the image for each wavelength.

In a biometric image processing apparatus 1-1 illustrated in FIG. 3, awhite lamp 111 is an example of the light source 11. The white lamp 111irradiates white light having wavelengths including the long-wavelengthand the short-wavelength on the biometric part 500. A first camera 121and a second camera 122 form an example of the imaging device (orimaging means). The returning light from the biometric part 500 is splitand separated into two paths by a prism 140. Light traveling in one ofthe two paths reaches the first camera 121 via a wavelength filter 141,and light traveling in the other of the two paths reaches the secondcamera 122 via a wavelength filter 142. The first camera 121 and thesecond camera 122 are arranged at positions so that optical axes thereofmatch, in order to avoid parallax.

The prism 140 and the wavelength filters 141 and 142 form an example ofa separator (or separating means) that separates the returning lightfrom the biometric part 500 into the long-wavelength component and theshort-wavelength component. The prism 140 splits and separates thereturning light from the biometric part 500 into two paths. Theband-limiting wavelength filter 141 cuts the short-wavelength componentof the returning light in one of the two paths, and transmits thelong-wavelength component of this returning light in the same path tosupply this long-wavelength component to the first camera 121. Theband-limiting wavelength filter 142 cuts the long-wavelength componentof the returning light in the other of the two paths, and transmits theshort-wavelength component of this returning light in the same path tosupply this short-wavelength component to the second camera 122.Accordingly, the first camera 121 supplies to the control device 13 thefirst image 50-1 illustrated in FIG. 2 that is captured based on thelong-wavelength component of the returning light. On the other hand, thesecond camera 122 supplies to the control device 13 the second image50-2 illustrated in FIG. 2 that is captured based on theshort-wavelength component of the returning light.

A semi-transparent mirror may be used in place of the prism 140 to splitand separate the returning light. However, it is more preferable to usethe prism 140 because a loss in the intensity of light is smaller forthe prism 140 when compared to that of the semi-transparent mirror.

FIG. 4 is a diagram illustrating an example of the biometric imageprocessing apparatus in a second embodiment. In FIG. 4, those parts thatare the same as those corresponding parts in FIG. 1 are designated bythe same reference numerals, and a description thereof will be omitted.In this embodiment, the light having the long-wavelength and the lighthaving the short-wavelength are simultaneously irradiated on thebiometric part, in order to observe the image for each wavelength.

In a biometric image processing apparatus 1-2 illustrated in FIG. 4, acamera 120 is an example of the imaging device (or imaging means). Inaddition, a filter 150 is an example of a separator (or separatingmeans) that separates the returning light from the biometric part 500into the long-wavelength component and the short-wavelength component.

FIG. 5 is a diagram illustrating an example of the filter. The filter150 illustrated in FIG. 5 includes a long-wavelength filter 151 or ashort-wavelength filter 152 for each pixel of a light receiving elementof the camera 120. In this example, the long-wavelength filter 151 andthe short-wavelength filter 152 are alternately arranged for each pixel,along both a horizontal direction and a vertical direction of the filter150. The long-wavelength filter 151 cuts the short-wavelength componentof the returning light, and transmits the long-wavelength component ofthe returning light to supply this long-wavelength component to thecamera 120. The short-wavelength filter 152 cuts the long-wavelengthcomponent of the returning light, and transmits the short-wavelengthcomponent of the returning light to supply this short-wavelengthcomponent to the camera 120. Accordingly, the camera 120 supplies to thecontrol device 13 the first image 50-1 illustrated in FIG. 2 that iscaptured via the long-wavelength filter 151. On the other hand, thecamera 120 supplies to the control device 13 the second image 50-2illustrated in FIG. 2 that is captured via the short-wavelength filter152. In this case, a number of effective pixels per image is reduced to½, however, only one camera 120 is required. Hence, compared to a casein which two cameras are used, it is possible to reduce the cost of thebiometric image processing apparatus 1-2. In addition, because only onecamera 120 is required, unlike the case in which two cameras are used,it is unnecessary to make adjustments to match the optical axes of thetwo cameras to avoid parallax.

FIG. 6 is a diagram illustrating an example of the biometric imageprocessing apparatus in a third embodiment. In FIG. 6, those parts thatare the same as those corresponding parts in FIG. 3 are designated bythe same reference numerals, and a description thereof will be omitted.In this embodiment, the light having the long-wavelength and the lighthaving the short-wavelength are sequentially irradiated on the biometricpart, in order to observe the image for each wavelength.

In a biometric image processing apparatus 1-3 illustrated in FIG. 6, thewhite lamp 111 and a wavelength filter 161 form an example of a firstlight source. In addition, a white lamp 112 and a wavelength filter 162form an example of a second light source. The band-limiting wavelengthfilter 161 transmits the long-wavelength component of the light emittedfrom the white lamp 11, to irradiate the long-wavelength component onthe biometric part 500. On the other hand, the band-limiting wavelengthfilter 162 transmits the short-wavelength component of light emittedfrom the white lamp 112, to irradiate the short-wavelength component onthe biometric part 500. The white lamp 111 and the white lamp 112 arecontrolled to alternately turn on by the controller 131 of the controldevice 13. In other words, the wavelength of the illumination light isswitched at the illumination end, by successively irradiating lighthaving the different wavelengths on the biometric part 500. For thisreason, no special restrictions are imposed on the observation end (thatis, the camera side). In this example, every time the light having thedifferent wavelength is irradiated on the biometric part 500, the camera120 captures one image of the biometric part 500.

FIG. 7 is a diagram illustrating an example of the biometric imageprocessing apparatus in a fourth embodiment. In FIG. 7, those parts thatare the same as those corresponding parts in FIGS. 4 and 6 aredesignated by the same reference numerals, and a description thereofwill be omitted. In this embodiment, the light having thelong-wavelength and the light having the short-wavelength aresequentially irradiated on the biometric part, in order to observe theimage for each wavelength.

In a biometric image processing apparatus 1-4 illustrated in FIG. 7, theconfiguration of the observation end (that is, the camera side) issimilar to that of the second embodiment illustrated in FIG. 4. On theother hand, the configuration of the illumination end is similar to thatof the third embodiment illustrated in FIG. 6. In other words, theconfiguration of the observation end (that is, the camera side) and theconfiguration of the illumination end of different embodiments may beappropriately combined.

FIG. 8 is a diagram illustrating an example of an RGB (Red, Green, Blue)filter. An RGB filter 150A illustrated in FIG. 8 may be used in place ofthe filter 150 illustrated in FIG. 5, in each of the second embodimentillustrated in FIG. 4 and the third embodiment illustrated in FIG. 6. Inother words, as illustrated on an enlarged scale for a top left part ofthe RGB filter 150A surrounded by a dotted line in FIG. 8, the RGBfilter 150A includes an R (Red) filter indicated by R, a G (Green)filter indicated by G, and a B (Blue) filter indicated by B, for eachpixel of the light receiving element of the camera. In this example, theR filter also functions as the long-wavelength filter 151, and the Bfilter also functions as the short-wavelength filter 152.

A complementary color filter, such as an YMC (Yellow, Magenta, Cyan)filter, may be used in place of a primary color filter, such as the RGBfilter. When the RGB filter is used, it is possible to more easilyreproduce sharp colors when compared to the case in which the YMC filteris used. On the other hand, when the YMC filter is used, it is possibleto obtain a brighter image because an amount of transmitting lightincreases due to light colors when compared to the case in which the RGBfilter is used.

Next, a description will be given of selection of wavelength of lighthaving mutually different wavelengths. FIG. 9 is a diagram illustratingan example of a multi-layered structure of skin. For example, accordingto Takaaki Maeda et al., “Monte Carlo Simulation of Spectral ReflectanceUsing Multilayered Skin Tissue Model”, Optical Review Vol. 17, No. 3,(2010), pp. 223-229, the skin has the multi-layered structureillustrated in FIG. 9. As illustrated in FIG. 9, the skin includes anepidermis layer 511 having a thickness of approximately 100 μm, a dermislayer 512 having a thickness of approximately 1.5 mm, and a subcutaneoustissue 513. The veins are located within the subcutaneous tissue 513that is 1.5 mm or deeper from the surface of the epidermis layer 511(that is, the skin surface).

When light is irradiated on the biometric part, the longer thewavelength of the light, the more the light penetrates the skin andreaches deeper inside the biometric part. For this reason, the lighthaving the long-wavelength is selected so that the light penetrates theskin and reaches inside the biometric part where the veins are located,that is, the inside that is 1.5 mm or deeper from the skin surface. FIG.10 is a diagram illustrating an example of a light penetration rate. InFIG. 10, the ordinate indicates a depth d (mm) from the skin surface,and the abscissa indicates a wavelength λ (nm) of the light. Inaddition, “2%”, “10%”, “20%”, “50%”, and “80%” added to plotsillustrated in FIG. 10 indicate the light penetration rates (%) of theamount of incoming light that penetrates the skin and reaches therespective depths inside the biometric part.

The light having the long-wavelength has a wavelength longer than 600nm. For example, according to Yoshinaga Aizu, “Skin Tissue MultilayeredStructure Modeling and Light Propagation Simulation”, Journal of theJapan Society of Mechanical Engineers (JSME), 2011.7, Vol. 114, No.1112, p. 39, it is preferable to select the wavelength of the lighthaving the long-wavelength (red to near-infrared wavelength range) to700 nm or longer.

On the other hand, the light having the short-wavelength has awavelength of 600 nm or shorter. The wavelength of the light having theshort-wavelength is selected so that the light is reflected or scatteredat the skin surface, and only penetrates the skin surface and can onlyreach a shallow region from the skin surface. It is preferable to selectthe wavelength of the light having the short-wavelength in a range of300 nm to 600 nm (blue to green wavelength range), for example.

The wavelength of the light irradiated on the biometric part may have asingle spectrum. However, in order to obtain light having a highmonochromaticity, a device such as a laser device may be used. Whenlight having a narrow bandwidth is to be emitted from a lamp (or bulb),an LED (Light Emitting Diode), or the like, a wavelength filter is usedto limit the band and the amount of light is decreased thereby. In eachof the embodiments described above, the two wavelengths that are usedsimply need to cause the light having the two wavelengths to reachmutually different depths of the biometric part. The two wavelengths ofthe light do not necessarily need to have a single spectrum, and thelight having the long-wavelength is possible to use light having abandwidth of 700 nm to 900 nm, for example. In addition, in a case inwhich a plurality of spectral light are combined and used, the lighthaving the short-wavelength may be a combination of light having awavelength of 400 nm and light having a wavelength of 500 nm, forexample.

Next, a description will be given of an operation of the computingdevice 132 that computes the difference between the two images. When thelight having the two wavelengths selected as described above isirradiated on the biometric part and the image of the biometric part iscaptured and observed, it is possible to obtain two images, namely, theimage caused by the light having the long-wavelength and the imagecaused by the light having the short-wavelength. The light having theshort-wavelength penetrates the surface of the biometric part but onlyreaches the shallow region of the biometric part. Hence, the imagecaused by the light having the short-wavelength only includes surfaceinformation related to a vicinity of the surface of the biometric part.On the other hand, the light having the long-wavelength penetrates thesurface of the biometric part and not only reaches the shallow region ofthe biometric part, but also reaches a deep region of the biometricpart. In other words, the returning light from the biometric part withrespect to the light having the long-wavelength includes returning lightfrom the shallow region close to the surface of the biometric part, andreturning light from the deep region further (or deeper) inside thebiometric part from the surface of the biometric part relative to theshallow region. For this reason, the image caused by the light havingthe long-wavelength includes both the surface information, and internalinformation related to the inside of the biometric part. The surfaceinformation included in and observed on the image caused by the lighthaving the long-wavelength corresponds to the surface informationincluded in and observed on the image caused by the light having theshort-wavelength. Accordingly, it is possible to extract the internalinformation of the biometric part, that is, an image of vein informationwithin the skin tissue, by subtracting from the image caused by thelight having the long-wavelength, the image in which the luminance ofthe second image caused by the light having the short-wavelength, isattenuated, according to the following formula (1). In the formula (1),I_(v) denotes an image of the vein information, I_(lw) denotes the imagecaused by the light having the long-wavelength, I_(sw) denotes the imagecaused by the light having the short-wavelength, and κ denotes acoefficient. The coefficient κ is a value that is less than 1, andrepresents the attenuation rate of the luminance of the image I_(sw)caused by the light having the short-wavelength. The attenuation rate atwhich the luminance of the image caused by the light having theshort-wavelength is attenuated may be determined according to theintensity of a component of the surface information (that is, surfacereflection light) included in the image caused by the light having thelong-wavelength.I _(v) =I _(lw) −κ·I _(sw)  (1)

As described above, when obtaining the difference between the imageI_(lw) caused by the light having the long-wavelength and the imageI_(sw) caused by the light having the short-wavelength, it is preferableto adjust and match the luminance of the image I_(sw) caused by thelight having the short-wavelength to the luminance of the image I_(lw)caused by the light having the long-wavelength. The coefficient κdepends on the wavelength range of the two illumination light used, aspectral sensitivity of the camera used, a spectral reflectivity of theskin, or the like. For the sake of convenience, it is assumed in thisexample that the light used, having the wavelengths including awavelength of 500 nm and a wavelength of 800 nm, is emitted from amonochromatic light source. It is also assumed that the coefficient κ isadjusted, so that luminance distributions (or histograms) of the imagescaused by the light having the two different wavelengths becomeapproximately the same.

The image that is observed represents a two-dimensional spatialdistribution of the returning light from the biometric part irradiatedwith the light. When light is irradiated on the biometric part and animage, caused by returning light from the biometric part after theirradiated light penetrates and reaches inside the biometric part to adepth z from the surface of the biometric part, is denoted by Img(z),the observed image becomes an overlap of images caused by returninglight from the respective depths. When the overlap of the images at thedepths z=a to b of the biometric part is represented by the followingformula (2), the observed image becomes an overlap I[0, dmax] of theimages at the depths from a depth 0 to a maximum penetration depth dmax.

$\begin{matrix}{{I\left\lbrack {a,b} \right\rbrack} = {\sum\limits_{z = a}^{b}{{Img}(z)}}} & (2)\end{matrix}$

From FIG. 10, in the case of the light having the wavelength of 500 nm,the light penetrating the biometric part and reaching the depth of 1 mmfrom the surface of the biometric part is less than 2% of the amount ofincoming light (that is, the light penetration rate is less than 2%),and almost all of the light returns from a depth of 1 mm or less. Forthis reason, the following formula (3) stands.I500[0, dmax]≈I500[0, 1 mm]  (3)

On the other hand, in the case of the light having the wavelength of 800nm, the light penetrates the biometric part and reaches the depth of 1mm or more from the surface of the biometric part. Hence, the followingformula (4) stands.I800[0, dmax]=I800[0, 1 mm]+I800[1 mm, dmax]  (4)

In the formulas (3) and (4), I500[0, 1 mm] and I800[0, 1 mm] are imagesobserved from the same layer of the biometric part, and only theluminance (that is, brightness) differs between the two images.Accordingly, the following formula (5) stands.I800[0, 1 mm]=κ·I500[0, 1 mm]  (5)

In addition, the formula (5) above can be replaced by the followingformula (6).I800[0, dmax]=κ˜I500[0, 1 mm]+I800[1 mm, dmax]  (6)

Further, the following formula (7) can be obtained from the formula (3)and the formula (6) described above.I800[0, dmax]≈κ·I500[0, dmax]+I800[1 mm, dmax]  (7)

The formula (7) described above is equivalent to the following formula(8).I800[1 mm, dmax]≈I800[0, dmax]−κ·I500[0, dmax]  (8)

In the formula (8), I800[1 mm, dmax] is an overlap of the images at thedepths of 1 mm or more, and partially includes a deep portion of thedermis layer 512. However, the image of the subcutaneous tissue 513where the veins are located is the image of the vein pattern.

From FIG. 10, in the case of the light having the wavelength of 800 nm,the light penetrating the biometric part and reaching the depth of 1 mmfrom the surface of the biometric part is 40% of the amount of incominglight (that is, the light penetration rate is 40%). In addition, thelight penetrating the biometric part and reaching the depth of 1 mm ormore from the surface of the biometric part is 60% of the amount ofincoming light (that is, the light penetration rate is 60%). When it isassumed that all of the light penetrating and reaching inside thebiometric part can be observed as the returning light, in the formula(4) described above, κ·I500[0, 1 mm] may be regarded as being 60% of thebrightness of I800[0, dmax], and thus, the coefficient κ that is usedmay be κ=0.6.

In a case in which polarization filters are arranged so that thepolarization direction at the illumination end and the polarizationdirection at the observation end are perpendicular to each other (thatis, the polarization filters are arranged perpendicularly to each other)to perform separation by polarization, the amount of light decreases. Inprinciple, the amount of light decreases by ½×½ or more (that is,decreases to 25% or less), and an effective amount of light attenuatesto 20% or less. However, in the example described above, the luminanceof the image I_(sw) caused by the light having the short-wavelength isadjusted to the luminance of the image I_(lw) caused by the light havingthe long-wavelength. In other words, the image in which the luminance ofthe image I_(sw) caused by the light having the short-wavelength isattenuated using the coefficient κ that is κ=0.6, is subtracted from theimage I_(lw) caused by the light having the long-wavelength. As aresult, the attenuation of the intensity of light is suppressed to 40%,and the intensity of light can be improved to approximately two timesthat obtainable in the case in which the separation by polarization isemployed. In addition, an SNR (Signal-to-Noise Ratio) can be improved toapproximately two times that obtainable in the case in which theseparation by polarization is employed.

In order to match the luminance distributions (or histograms) of theimages captured by the imaging device, the illumination intensity, thespectral sensitivity of the camera, the spectral reflectivity of theskin, or the like need to be adjusted. However, particularly thespectral reflectivity of the skin differs depending on the user, and itis difficult to perform the adjustments solely by the imaging device.For this reason, the luminance distribution may be adjusted using alevel adjustment of an image processing. By using the image processing,it becomes possible to cope with a slight change in input caused byindividual differences amongst users. In a case in which the spectralproperties greatly differ, the SNR of the image deteriorates due toluminance variations. In this case, it is desirable to perform theadjustments at the imaging device as much as possible, so that thespectral properties become uniform. Generally, the spectral property ofthe camera is determined by the light receiving element and the filter,and it may be practical to perform the adjustments according to a ratioof intensities of the illumination light.

In the example described above, the coefficient κ that is used is κ=0.6.However, in a case in which illumination light having other wavelengthsis used, or the illumination light is not from a monochromatic lightsource and is from a light source that emits light having a wavelengthin a certain wavelength range, the value of the coefficient κ isdetermined again since the difference between the penetration depths ofthe light becomes different. The value of the coefficient κ may bedetermined from computation, or based on a value actually measured withreference to a particular sample.

In order to make an inexpensive biometric image processing apparatus, itis conceivable to use a generally available RGB camera for the imagingdevice. In a case in which the light having the wavelength of 500 nm andthe light having the wavelength of 800 nm are irradiated on thebiometric part as in the example described above, and the image of thebiometric part is captured by the RGB camera, the light having thewavelength of 500 nm is observable only by a G-plane, however, the lighthaving the wavelength of 800 nm is observable in all of R-plane,G-plane, and B-plane (that is, by all of RGB planes). To be moreprecise, the light having the wavelength of 500 nm is slightlyobservable by the R-plane and the B-plane. This is because dyes used forthe RGB filter transmits the wavelengths in the near-infrared range. Inthis case, the image caused by the light having the short-wavelength of500 nm cannot be observed solely by the G-plane, and a combination ofthe image caused by light having the short-wavelength and the imagecaused by the light having the long wave-length is observed by theG-plane. For this reason, a ratio of the light having theshort-wavelength and the light having the long-wavelength, observable bythe G-plane, may be obtained from the spectral sensitivity of the Gfilter and the spectral sensitivity of the light imaging device, andthis ratio may be reflected to the value of the coefficient κ. Forexample, the image caused by the light having the short-wavelength maybe captured by a camera having the G-plane with a spectral sensitivityof 4:1 for the light having the wavelength of 500 nm and the lighthaving the wavelength of 800 nm. In this case, the image caused by thelight having the long-wavelength may be observed by the R-plane, and nocomponent of the light having the short-wavelength is included in theobserved image. First, the component of the light having thelong-wavelength is removed from the image observed by the G-plane, andan image of a G′-plane including only the component of theshort-wavelength is obtained from the following formula (9).G′=G−0.2R  (9)

Next, a vein image (or vein pattern image) V is obtained from thefollowing formula (10).

$\begin{matrix}\begin{matrix}{V = {R - {0.6G^{\prime}}}} \\{= {R - {0.6\left( {G - {0.2R}} \right)}}} \\{= {R + {0.12R} - {0.6G}}} \\{= {{1.12R} - {0.6G}}}\end{matrix} & (10)\end{matrix}$

A vein image (or vein pattern image) V′ may be obtained from thefollowing formula (11).V′=R−0.54G  (11)

Accordingly, even in a case in which a degree of separation of theobserved images is relatively low, it is possible to acquire a clearimage of the vein (vein pattern).

According to each of the embodiments described above, it is possible toobtain a clear biometric image. In addition, it is possible to use aninexpensive light source or an inexpensive camera. For this reason,compared to a case in which the polarization filters are used, forexample, it is possible to acquire the clear biometric image at a lowercost. In other words, it is possible to provide an inexpensive techniquefor capturing and acquiring a clear image of the internally scatteredlight eliminated of the surface reflection.

The description above use terms such as “determine”, “identify”, or thelike to describe the embodiments, however, such terms are abstractionsof the actual operations that are performed. Hence, the actualoperations that correspond to such terms may vary depending on theimplementation, as is obvious to those skilled in the art.

Although the embodiments are numbered with, for example, “first,”“second,” “third,” or “fourth,” the ordinal numbers do not implypriorities of the embodiments. Many other variations and modificationswill be apparent to those skilled in the art.

All examples and conditional language recited herein are intended forpedagogical purposes to aid the reader in understanding the inventionand the concepts contributed by the inventor to furthering the art, andare to be construed as being without limitation to such specificallyrecited examples and conditions, nor does the organization of suchexamples in the specification relate to a showing of the superiority andinferiority of the invention. Although the embodiments of the presentinvention have been described in detail, it should be understood thatthe various changes, substitutions, and alterations could be made heretowithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A biometric image processing apparatuscomprising: an imaging device configured to capture a first image of abiometric part that is irradiated with light having a first wavelength,and a second image of the biometric part irradiated with light having asecond wavelength shorter than the first wavelength; and a computingdevice configured to attenuate a luminance of the second image by anattenuation rate, and compute a third image by subtracting, from thefirst image, the second image having the luminance attenuated by theattenuation rate, wherein the attenuation rate of the second image isdetermined according to an intensity of a component of surfacereflection light included in the first image.
 2. The biometric imageprocessing apparatus as claimed in claim 1, wherein the computing devicedetermines the attenuation rate of the second image according to theintensity of the component of surface reflection light included in thefirst image, the first wavelength is longer than 600 nm, and the secondwavelength is 600 nm or shorter, and the third image is a vein image. 3.The biometric image processing apparatus as claimed in claim 1, furthercomprising: a light source configured to irradiate, on the biometricpart, light having wavelengths including the first wavelength and thesecond wavelength; and a separator configured to separate returninglight from the biometric part into a first component having the firstwavelength and a second component having the second wavelength, whereinthe imaging device includes a first camera configured to capture thefirst image based on the first component separated by the separator, anda second camera configured to capture the second image based on thesecond component separated by the separator.
 4. The biometric imageprocessing apparatus as claimed in claim 3, wherein the separatorincludes a prism configured to receive the returning light from thebiometric part, a first optical wavelength filter configured to transmitthe first component of the returning light separated by the prism, and asecond optical wavelength filter configured to transmit the secondcomponent of the returning light separated by the prism, wherein thefirst camera and the second camera are arranged so that optical axesthereof match.
 5. The biometric image processing apparatus as claimed inclaim 1, further comprising: a light source configured to irradiate, onthe biometric part, light having wavelengths including the firstwavelength and the second wavelength; a first filter configured totransmit a first component having the first wavelength and to cut asecond component having the second wavelength, of returning light fromthe biometric part; and a second filter configured to transmit thesecond component and to cut the first component, of the returning lightfrom the biometric part, wherein the imaging device includes a cameraconfigured to capture the first image based on the first componenttransmitted through the first filter, and to capture the second imagebased on the second component transmitted through the second filter. 6.The biometric image processing apparatus as claimed in claim 5, furthercomprising: an RGB filter that includes the first filter and the secondfilter.
 7. The biometric image processing apparatus as claimed in claim1, further comprising: a first light source configured to irradiate thebiometric part with light having the first wavelength; a second lightsource configured to irradiate the biometric part with light having thesecond wavelength; and a controller configured to alternately turn onthe first light source and the second light source, wherein the imagingdevice captures the first image based on a component having the firstwavelength of the returning light from the biometric part, while thecontroller turns on the first light source, and captures the secondimage based on a component having the second wavelength of the returninglight from the biometric part while the controller turns on the secondlight source.
 8. The biometric image processing apparatus as claimed inclaim 1, further comprising: a first light source configured toirradiate the biometric part with light having the first wavelength; asecond light source configured to irradiate the biometric part withlight having the second wavelength; a first filter configured totransmit a first component having the first wavelength and to cut asecond component having the second wavelength, of returning light fromthe biometric part; and a second filter configured to transmit thesecond component and to cut the first component, of the returning lightfrom the biometric part, wherein the imaging device includes a cameraconfigured to capture the first image based on the first componenttransmitted through the first filter, and to capture the second imagebased on the second component transmitted through the second filter. 9.The biometric image processing apparatus as claimed in claim 8, furthercomprising: an RGB filter that includes the first filter and the secondfilter.
 10. The biometric image processing apparatus as claimed in claim1, wherein the computing device normalizes a luminance level of thesecond image according to a luminance level of the first image.
 11. Abiometric image processing method comprising: capturing, by an imagingdevice, a first image of a biometric part that is irradiated with lighthaving a first wavelength, and a second image of the biometric partirradiated with light having a second wavelength shorter than the firstwavelength; and computing, by a computing device, the second imagehaving a luminance thereof attenuated by an attenuation rate, and athird image by subtracting, from the first image, the second imagehaving the luminance attenuated by the attenuation rate, wherein theattenuation rate of the second image is determined according to anintensity of a component of surface reflection light included in thefirst image.
 12. The biometric image processing method as claimed inclaim 11, wherein the computing determines, by the computing device, theattenuation rate of the second image according to the intensity of thecomponent of surface reflection light included in the first image, thefirst wavelength is longer than 600 nm, and the second wavelength is 600nm or shorter, and the third image is a vein image.
 13. The biometricimage processing method as claimed in claim 11, further comprising:irradiating on the biometric part, by a light source, light havingwavelengths including the first wavelength and the second wavelength;and separating, by a separator, returning light from the biometric partinto a first component having the first wavelength and a secondcomponent having the second wavelength, wherein the capturing capturesthe first image by a first camera of the imaging device based on thefirst component separated by the separating, and captures the secondimage by a second camera of the imaging device based on the secondcomponent separated by the separating.
 14. The biometric imageprocessing method as claimed in claim 11, further comprising:irradiating on the biometric part, by a light source, light havingwavelengths including the first wavelength and the second wavelength; ofreturning light from the biometric part, transmitting a first componenthaving the first wavelength and cutting a second component having thesecond wavelength, by a first filter; and of the returning light fromthe biometric part, transmitting the second component and cutting thefirst component, by a second filter, wherein the capturing captures, bya camera of the imaging device, the first image based on the firstcomponent transmitted through the first filter, and the second imagebased on the second component transmitted through the second filter. 15.The biometric image processing method as claimed in claim 14, which usesan RGB filter that includes the first filter and the second filter. 16.The biometric image processing method as claimed in claim 11, furthercomprising: irradiating the biometric part, by a first light source,with light having the first wavelength; irradiating the biometric part,by a second light source, with light having the second wavelength; andcontrolling, by a controller, the first light source and the secondlight source to alternately turn on the first light source and thesecond light source, wherein the capturing captures, by a camera of theimaging device, the first image based on a component having the firstwavelength of the returning light from the biometric part, while thecontroller turns on the first light source, and the second image basedon a component having the second wavelength of the returning light fromthe biometric part while the controller turns on the second lightsource.
 17. The biometric image processing method as claimed in claim11, further comprising: irradiating the biometric part, by a first lightsource, with light having the first wavelength; irradiating thebiometric part, by a second light source, with light having the secondwavelength; of returning light from the biometric part, transmitting afirst component having the first wavelength and cutting a secondcomponent having the second wavelength, by a first filter; and of thereturning light from the biometric part, transmitting the secondcomponent and cutting the first component, by a second filter, whereinthe capturing captures, by a camera of the imaging device, the firstimage based on the first component transmitted through the first filter,and the second image based on the second component transmitted throughthe second filter.
 18. The biometric image processing method as claimedin claim 17, which uses an RGB filter that includes the first filter andthe second filter.
 19. The biometric image processing method as claimedin claim 11, wherein the computing includes normalizing, by thecomputing device, a luminance level of the second image according to aluminance level of the first image.
 20. The biometric image processingapparatus as claimed in claim 1, wherein the computing device computesthe third image according to a formula I_(v)=I_(lw)−κ·I_(sw), whereI_(v) denotes the third image, I_(lw) denotes the first image, I_(sw)denotes the second image, and κ denotes a coefficient that is less than1 and represents the attenuation rate of the luminance of the secondimage I_(sw) caused by the light having the second wavelength,determined according to an intensity of a component of surfacereflection light included in the first image I_(lw) caused by the lighthaving the first wavelength.